Inhibition of the Corrosion of Mild Steel in Acidic Environment Using Azadirachta Indica (NEEM) Leaf Extract as Inhibitor

ABSTRACT

The inhibition of corrosion of mild steel by Azadirachta indica leaf extract in acidic environments of HCl and HNO3 from pH1 to pH6 has been investigated using the weight loss technique. The Azadirachta indica leaf extract is the inhibitor and the mild steel used was in the form of coupons. The investigations were carried out at ambient temperature. The optimum concentration of the extract used for the experimentation was obtained by soaking 200g of ground fresh Azadirachta indica leaves in 250mls of cold water (i.e. 0.8g Azadirachta indica leaf/ml of water) for 30minutes. The inhibitor reduced the corrosion rate in HNO3 environment by over 70% at pH2 within 6hrs while the corrosion rate in HCl was reduced by about 60% in pH3 within 6hrs. The inhibition efficiency was up to 85% in the HNO3 environment and 74% with the HCl. The efficiency of inhibition increased with increased exposure time but decreased with increase in pH of the environment. The inhibition of mild steel corrosion in the acid environments by Azadirachta indica leaf extract can be attributed to the adsorption of the inhibitor constituents (tannins, trisulphides, tetrasulphides etc) on the surface of the coupons. The data from the experimentation are consistent with Flory-Huggins adsorption isotherm and gives a standard free energy of adsorption ∆G0ads = -10.3kJ/mol which suggests physical adsorption mechanism, since this value is less than the threshold value for physical adsorption. The adsorption process in all cases was largely completed within the first nine hours of experimentation initiation with the resultant lowering and maintenance of the corrosion rate at a near constant value. This phenomenon may facilitate batch inhibitor application rather than the continuous application in facilities used for containing or transporting corrosive fluids.

Key words: Azadirachta indica leaf, mild steel, inhibition, corrosion rate, inhibition efficiency, adsorption isotherm.